Feel you've been here before?

That strange familiarity you can't quite put your finger on? You're suffering from what we spin-doctors call "deja-vow".

Large numbers of voters were complaining of the condition yesterday as Jim Murphy unveiled his latest bid to replace the SNP as the party of patriotic social democracy.

Keeper of the Vow Gordon Brown has agreed to storm more barns across Scotland in the coming months, in his latest final farewell tour. Mr Murphy will be unveiling his own Irn Bru 2 event accompanied by a band of specially selected egg-throwing cybernats.

All we need now is a posse of poorly-briefed hacks to come stomping over the border looking for anti-English racists. And tabloid stories claiming that voting SNP in May could spark another Great Depression, halt cancer cures, boost terrorism and increased divorce rates.

Well, why not. The Vow wheeze worked last time, sort of. Labour's electoral plight in Scotland - despite the expected boost in tomorrow's Lord Ashcroft constituency polls - looks so desperate that they might as well try anything. But it is just a little early.

Victory-in-the-jaws-of-defeat initiatives work best when they are deployed later in the day - preferably at the last minute. To be talking about new vows now, offering what looks like another version of Smith reforms, is likely to leave voters more than a little confused. Labour reportedly blocked welfare devolution during the Smith Commission.

Anyway, if Smith was supposed to have created true home rule, or "as near to federalism as possible in a country where 85 per cent of he people live in one part of it", as Gordon Brown put it, how can there now be another home rule settlement so soon after it?

Since 2009 we've had Calman, Calman plus, Scotland Act, Smith, Smith plus...where will it end? If the drift of policy seems irrevocably in one direction then people might get the impression that there is only one destination: independence or something very close to it.

At the very least there is a law of diminishing vows. Of course, Mr Brown said yesterday this wasn't really a new Vow, just the last one being extended. But it is all part of the process of being more Nat than the Nats. The danger of aping your political enemy is that you validate their programme and sow confusion in your own ranks.

What are loyal Labour voters supposed to think? With all the chops and changes on tax, tuition fees, universal benefits, nationalisation, fracking, devo max they must wondering whether they're coming or going. Giving Scotland further powers to set benefits irrespective of UK welfare policy is either phoney or a serious step along the road to devolution max. Remember, welfare is still supposedly reserved to Westminster.

The Scottish Parliament, we're told, is to be given powers by Labour to top up any benefits it wishes to increase, and to reject benefit reforms it doesn't like. We are told a bedroom tax "could never happen again" under Labour's new proposals. This is all very well, but is it coherent policy making?

There is a danger of a benefits arms raced developing here in which the SNP and Labour end up giving the impression that the Scottish parliament can set significantly higher benefit levels - pensions, child benefit, incapacity benefits etc. - without being given the means to pay for them.

In future the Scottish Parliament is going to be self-financing a large part of its activities on the basis of one highly inflexible tax: income tax. This means that rash promises on benefits will read over directly onto taxation on middle income Scottish families. With a declining tax base and an ageing population it is going to be difficult enough to finance the Scottish government's programmes without adding huge potential costs on top.

Scotland is not an independent country like Norway with an advanced economy and a healthy oil fund. It remains a dependent region of the United Kingdom with poor business infrastructure and little true economic autonomy.

Here is a vow that all the parties should take. Refrain from making unrealistic promises on the basis of powers that Holyrood does not have, and on revenue that has yet to be found. Otherwise they might find the voters delivering a few oaths of their own.